In recent years, while wireless communication technology has been developed vigorously worldwide, the wireless communication industry faces a series of problems that are difficult to solve, such as various communication systems existing simultaneously, various communication standards competing sharply, and shorting of wireless frequency resources, and the like. In order to help solve these problems, it is desired that digitalization of a wireless communication system may be as close to Radio Frequency (RF) front end as possible. Also, it is desired that the fundamental function elements of various wireless communications are implemented by software because modifying only the system software would make the supporting of the other communication systems easily. These features have become a necessity of wireless communication systems, particularly involving in broadband wireless communication systems. Further, in broadband wireless communication systems, it is also desired to have super linearity and wide dynamic range in broadband transmitting channels.
In order to utilize frequency resources more effectively, modulation methods with high utilization efficiency of the frequency spectrum, such as SSB, QPSK, 8PSK, and etc., are employed by many communication systems. Not only is the phase of a carrier modulated by these modulation methods, but also the amplitude of the carrier is modulated by these modulation methods. Therefore, non-constant envelope modulation signals with a larger peak to average ratio are produced by these modulation methods. However, for the constant envelope modulation, such as GMSK, if multi-carrier technology is used, a larger envelope fluctuation may also be produced by utilizing a signal combiner to combine the signals of a plurality of carriers into a broadband signal. A higher linearity requirement may be raised for broadband transmitting channels with a larger peak to average ratio, particularly, for digital to analogue (D/A) converters, broadband transmitters (BTXs) and broadband power amplifiers. Generally, semiconductor devices have an unavoidable non-linearity characteristic. When a broadband transmitter is operating at a non-linear region, it will produce an inter-modulation component, thereby causing signals interfering with each other in-band.
As discussed earlier, it is commonly desired to improve communication system linearity in a broadband communication system. Three methods have been used to solve the linearity problems. One is to use a suitable super linear semiconductor device and to design a broadband transmitter that accords with the performance requirements of the super linear semiconductor device. However, this method is expensive and involves many technology difficulties. The second method is to reduce the power of the entire transmitting channel, such that the transmitting channel may operate in a linear region. However, this method significantly decreases the operating efficiency of the broadband communication system. The third method is to employ a linearization technology, that is, proper peripheral circuits are used to correct the non-linearity of the transmitting channel, thereby presenting a linear amplifying effect for the input signals by the circuits in its entirety. This method may avoid using difficult technology for manufacturing semiconductor devices and allows the use of devices with relatively low cost. In addition, this method not only allows a variety of forms, but also provides flexibility in selecting devices. Thus, the third method has been considered as the most suitable method at present.
In a linearization method, predistortion technology is the most simple and effective method. In a predistortion system, signals are corrected firstly by a predistorter, then sent to a power amplifier for amplifying and outputting. The signal distortion characteristic produced by the predistorter is opposite to that produced by a transmitting channel, thereby a distortion component of the transmitting channel is canceled out so as to obtain a non-distortion output.
A typical narrow band predistortion system is shown in FIG. 1. The baseband signals are corrected by a predistorter first. Then, the signals are quadrature modulated, D/A converted by a D/A converter, up-converted by an up-converter, amplified by a power amplifier, and finally outputted by an antenna. Meanwhile, a part of the signals outputted from the power amplifier are fed back by a coupler and converted into baseband signals through a band-pass filter (not shown), down-converted by a down-converter, A/D converted by an A/D converter, demodulated by a quadrature demodulator (not shown), and sent to an error processor. The other input of the error processor is a delayed baseband input signal. The difference between the two signals are compared in the error processor to output a signal to the predistorter. The parameters of the predistorter are updated. An adaptive function can be realized in the error processor. However, in the broadband communication systems, the delay of a frequency component of different signals on the transmitting channels cannot be approximated to the same value. Comparing the differences between the input signals and output signals with a simple delay is not suitable for solving the non-linearity problem of the transmitting channels of the broadband communication systems. Thus, there is a need for a predistortion apparatus and method for compensating the non-linearity of transmitting channels of broadband communication systems.